I. Field of the Invention
The present invention is directed generally to pulsed radar receiving transmitters and, more particularly, to an improved radar receiver transmitter using a single oscillator frequency shift keyed system.
I. Description of the Related Art
Because of different characteristics of high and low frequency RF signals with respect to RF transmission and internal signal processing, most high gain systems utilize some type of a frequency-changing scheme. In such a system, the echo radar signal, in the case of a radar system, or other incoming signal of interest to be received is brought in from the antenna, amplified and mixed with a locally generated signal differing in frequency from it by a predetermined constant amount spoken of as the intermediate frequency. The intermediate frequency (IF) is a much lower frequency than the transmitted/received signal and so it is easier to amplify and tune for use as a video signal output, or the like. A signal processing system employing a frequency changer and intermediate-frequency amplifier for the mixer output is generally known as a superheterodyne system.
In the past, there have been systems which have been developed utilizing zero IF. In these systems, the signal of interest is transmitted and the echo received at one frequency and a local oscillator is provided which runs at the same frequency. Because the frequencies are the same it is called a zero IF system. It is well known that it is generally more difficult to accurately process the high frequency signal components in a receiver; and, therefore, because the zero IF systems work with the full frequency signal, those systems must be designed to work with relatively low frequency transmissions. Zero IF systems further require IF amplifiers with large low frequency capacitors and have difficulties with regard to leakage signals and pulses if one attempts to use them with a system equipped with a single antenna and a circulator, thereby making them undesirable from these standpoints.
Because of difficulties encountered with the zero IF systems and certain other benefits obtained with superheterodyne systems, systems utilizing an offset intermediate frequency are preferred. Known systems of this type typically are provided with two oscillators. One of the oscillators operates at the high frequency of the radar pulse or other signal transmitted and received external to the signal processing environment. The other oscillator generates another high frequency local signal offset from the transmitter signal by a small known amount. In order for the intermediate frequency to remain constant, however, these two oscillators must frequency track or remain constant in frequency output relative to each other over wide temperature, humidity and other environmental ranges. They also have to remain fairly stable. To create oscillators with the required characteristics or to add compensation or correcting circuitry to the system to accomplish tracking over a preferred fairly narrow bandwidth results in a very complicated and costly device. As an alternative, some of these devices have utilized a very wide band with intermediate frequencies so that if the IF frequency varies a little bit over a temperature range, for example, the system will function reasonably well. Sensitivity is lost, however, because the wider bandwidth allows more noise to go through the system.
Problems of this nature are particularly noticeable in systems which must operate accurately over a fairly wide range of environmental conditions with relatively great accuracy. Devices of this type include, for example, radar altimeters. Accordingly, there remains a need for a relatively simple, yet stable system which combines the tracking advantages of a single frequency with the sensitivity and discrimination of a superheterodyne system.